The increasing utilization of burners that achieve lower and lower levels of NOx emissions oftentimes does not promote complete combustion of the fuel. This can result in production of higher levels of CO, which is undesirable. Also, new high efficiency or low emission burners utilize small ports and oftentimes act as filters. Over time, such burners may become covered with or subjected to the presence of lint, dirt, oils and the like that are normally found in residential and commercial environments. As the small ports fill with such extraneous material, the levels of CO may increase.
Economic and effective solutions to the presence of CO generated by water heaters have not been entirely successful.
Also, the HVAC, water heater and small appliance industries are continuously updating product designs to meet more stringent combustion emission, energy efficiency, and safety (flammable vapor and lint, dirt and oil resistant) requirements. To achieve these requirements, new burner technologies are being utilized that replace older burner technologies.
The new burner technologies typically utilize significantly reduced port size to achieve desired performance improvements. Unfortunately, these smaller ports may collect foreign materials (such as lint, dirt or oil aerosols) present in the air drawn into the combustion system. As these materials collect on or in the burner ports over time, the performance of the burner may degrade, resulting in higher emissions of carbon monoxide, nitrogen oxides, and overall lower efficiency.
One way of reducing this problem is to utilize a filter on the incoming combustion air. However, such filters add extra cost and often add significant pressure drops that either cannot be overcome or necessitate use of fans or blowers.
Some of the new burner technologies unload a portion of the foreign materials that collect in the ports during burner ignition. The unloading process is due to the large, short duration local pressure and velocity gradients achieved during initial ignition of the combustion gas (typically natural gas or propane). The local pressure and velocity gradients create pressure and friction forces that dislodge some portion of the foreign materials from the burner ports, allowing the burner to return to, or approach, the original “as new” condition.
Additionally, in some of the new burner designs, the flame holder material containing the small ports operates at a high temperature while the burner is operating. When the burner is shut off, the flame holder temperature rapidly drops, approaching ambient temperature. The rapid rise and fall of the flame holder temperature creates thermal movement and stresses in the flame holder, causing the foreign materials to loosen or fall off.